[Feature Request]: Add XML - redesign transform to allow complex XML structures

### What would you like to happen?

A missing feature in the **Add XML** transform is the ability to merge XML fragments in groups (see discussion #5395 and bug #5396).

The ideal way to solve this should be implementing similar features to those already present in the **Enhanced JSON output**, notably:
- the ability to define grouping fields (and their XML name, i.e. the "root" tag for the fragment), so that the other fields can be nested with their own XML
- the ability to define a non-grouping field as an "XML fragment", so that it treated as it is rather than escaping the `<` and `>` symbols with `<` and `>`
- the ability to pretty print the XML output (as mentioned in #2385)

### Issue Priority

Priority: 2

### Issue Component

Component: Hop Gui, Component: Transforms

Start by reading the Add XML transform requirements alongside the existing Enhanced JSON output features, then review discussion #5395 and bug #5396 for the expected grouping behavior. Done means the transform supports grouped XML fragments, unescaped XML fragment fields, and pretty-printed output in the Hop GUI transforms component.

Written by the indexing model from the issue text.